terrier types with the bull terriers in particular are renound and famous for hating cats and will kill them. However there are many thousands of family staffies living all over the country in perfect harmony with the cats of the house.
What i've often heard of dogs (not just staffies) is they can be totally 100% with the cats in "their family" (which would include leo if they see each other often enough) but maybe they wouldnt be so trust worthy with a totally strange cat who perhaps wondered into the garden.
Bear in mind also that your sisters dog is PART staffie (so could also be part something else that are renound for adoring cats
). Has she had him since a puppy ?? if so she will have a very good idea and gut feeling of the dogs reactions and should be very intune with his body language. (how old is alfie ?)
It doesnt sound like leo is afraid of him in the slightest but i understand your concern, i too was with cody and my greys for a start (the dogs came into the house AFTER my 2 dsh cats who were very large fully grown adults, however cody was last in and although not a tiny 8 wks old kitten he was still strange and smaller ) however strict supervision all was well and the 3 now all sleep shut in the kitchen at night.
I'd get your sister to deliberately take alfie around more often, sounds like you yourself are actually a wee bit afraid and therefore you need to gain YOUR trust in your dog , you being wary will unerve alfie so please be aware of that, dogs need people to be confident !!! Dont leave cat & dog alone together even if you are just going to the loo but with close supervision they could be great friends and will only add to leos confidence, plus he should hopefully have another 20 years ahead of him and you never know you may have a dog of your own in that time
Just read your last post again and it actually seems like alfie and leo have no problem , just you lol (perfectly understandable though) Remember leo has far far quicker reflexes than alfie and also they will communicate with each other through body language, plus with you and or your sis monitoring to assess body language / growling, ears, eyes, tail, stance etc etc i am sure this could become a good friendship. I actually suggest you taking alfie on some walks etc and in different situations and you can assess his response and learn about him.
Also maybe take some of the toys away when alfie is around ?